The present invention relates to an eyeglass lens grinding apparatus which grinds an eyeglass lens so that the lens fits into an eyeglass frame, and also to a method for the same.
An eyeglass lens grinding apparatus is known in which a distance between the rotary shaft holding a subject lens and each of rotary shafts of grinding wheels rotated by motors is changed so that the subject lens is processed with pressing an edge of the lens against the grinding wheels suitable for the intended process, such as a rough grinding wheel, a finishing grinding wheel, and a mirror-polishing grinding wheel. As the motors for rotating the grinding wheels, used are AC motors in which the rotational speed cannot be changed. Therefore, the rotational speed of each grinding wheel is always constant.
When the rotational speed of each grinding wheel is constant, however, there arise the following problems. In rough and finishing processes for a plastic lens, for example, as the rotational speed of a grinding wheel is higher, the grinding amount can be made larger so that the processing time period is shorter. In contrast, in a mirror-polishing process, when the rotational speed of the grinding wheel is too high, grinding heat is excessively generated, and hence the process surface is easily burned. Furthermore, the surface is hardened and the mirror-polishing process is hardly performed on the surface.
In a mirror-polishing process of a lens made of polycarbonate which is a thermoplastic-material, because of the properties of the material, a rough process, a finishing process, and a mirror-polishing process using a mirror-polishing grinding wheel are performed while not supplying a grinding fluid, and thereafter the grinding fluid is finally supplied so as to give a gloss. In the stage of the mirror-polishing process which is performed without supplying the grinding fluid, when the rotational speed of the grinding wheel is high, grinding heat is excessively generated and the process face melts. Consequently, the quality of the process face is lowered. In contrast, in a mirror-finishing process stage in which the mirror-finishing process is performed while supplying the grinding fluid, when the rotational speed of the grinding wheel is too low, the friction heat in the process face is deficiently generated and hence the process face is not softened. As a result, the process face cannot be processed.
When a process for a glass lens using a rough grinding wheel or a finishing grinding wheel is performed at a high rotational speed in the same manner as the case of a plastic lens, grinding heat is excessively generated and hence the lens easily cracks or breaks.